Electrolytic tin plating solution

ABSTRACT

An electrolytic tin plating solution contains a compound serving as a source of supply of tin ions and an unsaturated aldehyde compound having a heterocyclic group.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2018-177797 filed on Sep. 21, 2018, the entire disclosure of which isincorporated by reference herein.

BACKGROUND ART

The present disclosure relates to an electrolytic tin plating solution,and more particularly relates to an electrolytic tin plating solutionwhich can reduce voids formed after reflow and which is suitable forforming a bump.

A connection bump is provided on a semiconductor chip. A solder ball orany other suitable structure is used as a connection bump. However,miniaturization of semiconductor chips increases the difficulty in usinga known solder ball as the connection bump. Although microballs having adiameter of about 100 μm may also be used, finer design rules have beenrequired. Attention has been given to the formation of a bump platedwith tin (Sn) or a Sn alloy (see, for example, Japanese UnexaminedPatent Publication No. 2016-106181).

If a minute bump is formed by plating, a plurality of films having athickness of several tens of micrometers and independent of one anotherneed to be uniformly deposited in the several-tens-of-micrometer range.To improve the joint reliability, voids to be formed need to be reduced.Furthermore, even if a step is present on a BVH (blind via hole) formingpart of a film, a uniform film needs to be deposited.

SUMMARY OF THE INVENTION

However, an electrolytic tin plating solution suitable for forming aminute bump has not been present yet. In particular, even if a step isformed on the BVH, a uniform bump needs to be formed without any void.

The present disclosure attempts to provide an electrolytic tin platingsolution suitable for forming, e.g., bumps.

An electrolytic tin plating solution according to one aspect of thepresent disclosure contains a compound serving as a source of supply oftin ions and an unsaturated aldehyde compound having a heterocyclicgroup. Since the electrolytic tin plating solution according to theaspect contains the unsaturated aldehyde compound having theheterocyclic group, a uniform film containing grains having a smallgrain size can be formed.

In the electrolytic tin plating solution according to the aspect, theheterocyclic group may be a five- or six-membered heterocyclic groupcontaining at least one of an oxygen atom, a nitrogen atom, or a sulfuratom.

In the electrolytic tin plating solution according to the aspect, acontent of the unsaturated aldehyde compound having the heterocyclicgroup may be from 0.01 mmol/L to 10 mmol/L.

In the electrolytic tin plating solution according to the aspect, grainsto be deposited may have a maximum grain size of 10 μm or smaller.

An electrolytic tin plating solution according to an aspect of thepresent disclosure allows a uniform film containing grains having asmall grain size to be formed.

DESCRIPTION OF EMBODIMENTS

An electrolytic tin plating solution according to an embodiment containsan unsaturated aldehyde compound having a heterocyclic group(hereinafter referred to as a heterocycle-containing unsaturatedaldehyde compound). The heterocycle-containing unsaturated aldehydecompound functions as a crystal regulator, which reduces the grain sizeof Sn deposit by electrolytic plating. Reducing the grain size allowsfor formation of a dense Sn deposit having less voids. Further, theelectrolytic tin plating solution containing the heterocycle-containingunsaturated aldehyde compound allows for formation of a Sn deposithaving a uniform thickness also on a BVH having a step.

The unsaturated aldehyde is aldehyde having a straight chain or abranched chain of molecules each having one or more unsaturated bonds,and may have a group other than a heterocyclic group. Examples of theunsaturated aldehyde include acrolein, methacrolein, crotonaldehyde,2-methylcrotonaldehyde, 2-ethylcrotonaldehyde, 2-ethylacrolein,2-ethyl-2-hexenal, citronellal, 2,3-dimethyl-2-propenal, undecylenicaldehyde, 4-heptenal, 2-hexenal, 2-undecenal, 2-nonenal,2-formylpropenenitrile, 3-ethoxy-2-methyl-2-propenal,4-hydroxy-2-nonenal, citronellyloxyacetaldehyde, 2-heptenal, 2-octenal,2-decenal, 2,4-nonadienal, 2,6-nonadienal, 2,4-octadienal,2,4-decadienal, and farnesal. The unsaturated aldehyde may include astereoisomer, which may be any stereoisomer or include variousstereoisomers.

The heterocyclic group should not be specifically limited, but may be afive-, six-, or seven-membered nitrogen-, oxygen-, or sulfur-containingheterocyclic group, or a nitrogen-, oxygen-, or sulfur-containingpolycyclic group. Examples of the heterocyclic group or the polycyclicgroup include a five-membered ring heterocyclic group, such as apyrrolidine group, a pyrrole group, a tetrahydrofuran group, an oxolanegroup, a furan group, a tetrahydrothiophene group, a thiolane group, athiophene group, an imidazole group, a pyrazole group, an imidazolinegroup, an oxazole group, a thiazole group, a thiazolidine group, atriazole group, a tetrazole group, a dioxolane group, an oxadiazolegroup, and a thiadiazole group, a six-membered ring heterocyclic group,such as a piperidine group, an azinane group, a tetrahydropyran group,an oxane group, a tetrahydrothiopyran group, a pyridine group, a pyranegroup, a thiopyran group, a pyrimidine group, a pyrazine group, apyridazine group, a thiazine group, a morpholine group, a dioxane group,a dithiin group, a thiomorpholine group, a trithiane group, and adithiazine group, a seven-membered ring heterocyclic group, such as athiazepine group, and a polycyclic group, such as an indole group, anisoindole group, an indolizine group, a benzimidazole group, abenzotriazole group, a purine group, a quinoline group, an isoquinolinegroup, a quinazoline group, a quinoxaline group, a cinnoline group, aphthalazine group, a chromene group, an isochromene group, abenzodioxole group, a benzodioxan group, a benzoxazole group, abenzothiazole group, a pteridine group, a phenothiazine group, aphenanthridine group, and a thianthrene group. The heterocyclic groupmay include a constitutional isomer and a stereoisomer, which may be anyisomer or include various isomers.

These unsaturated aldehydes and these heterocyclic groups may beoptionally combined together. Examples of the resultant compound include3-(2-furyl)acrolein, 2-methyl-3-(2-furyl)propenal,3-(5-nitro-2-furyl)acrolein, (4-pyridyl)acrolein,1,3-benzodioxole-5-acrolein,3-[3-(4-fluorophenyl)-1-isopropylindol-2-yl]acrolein,5-hydroxytetrahydrofuran-2-acrolein, 5-(2-furyl)-2,4-pentadienal,2-formyl-3-(2-furyl)propenenitrile, 2-cyano-3-(2-furyl)propenal,3-(4-pyridyl)propenal, pyridine-3-propenal, 3-(1H-indole-3-yl)propenal,3-(3,4-diethyl-2-pyrrolyl)propenal, 3-(2-thienyl)acrolein, and3-(isoindoline-2-yl)propenal. In particular, (2-furyl)acrolein,(4-pyridyl)acrolein, and 1,3-benzodioxole-5-acrolein are preferably usedfor reasons such as cost, ease of availability, and stability. Theheterocycle-containing unsaturated aldehyde compound may include aconstitutional isomer and a stereoisomer, which may be any isomer orinclude various isomers. One or more types of the heterocycle-containingunsaturated aldehyde compound may be contained in the plating solution.

The concentration of the heterocycle-containing unsaturated aldehydecompound in the electrolytic tin plating solution according to thisembodiment is preferably from 0.01 mmol/L to 10 mmol/L and morepreferably from 0.1 mmol/L to 1 mmol/L, to maintain a small grain sizeand to make the film deposited on the step uniform.

To reduce the formation of voids, the grain size of a maximum one ofgrains forming the Sn deposit formed using the electrolytic tin platingsolution according to this embodiment is preferably 10 μm or smaller andmore preferably 9 μm or smaller. The maximum one of the grainspreferably has a smaller grain size. However, an actually possible grainsize range of the maximum grain is preferably 0.1 μm or larger, morepreferably 0.5 μm or larger, still more preferably 1 μm or larger, andyet more preferably 3 μm or larger. A minimum one of the grainspreferably has a smaller grain size. However, an actually possible grainsize range of the minimum grain is, but not limited to, preferably 0.1μm or larger, more preferably 0.5 μm or larger, and still morepreferably 1 μm or larger.

The electrolytic tin plating solution according to this embodimentcontains a compound serving as a source of supply of tin (Sn) ions, inaddition to the heterocycle-containing unsaturated aldehyde compound.Examples of the compound serving as the source of supply of Sn ionsinclude a tin salt. In particular, a first tin salt (tin salt (II)) anda second tin salt (tin salt (IV)) are preferably used.

Non-limiting examples of the first tin salt (tin salt (II)) include tin(II) alkanesulfonate, such as tin (II) methanesulfonate, organic tin(II) sulfonate, such as tin (II) alkanolsulfonate such as tin (II)isethionate, tin (II) sulfate, tin (II) fluoroborate, tin (II) chloride,tin (II) bromide, tin (II) iodide, tin (II) oxide, tin (II) phosphate,tin (II) pyrophosphate, tin (II) acetate, tin (II) citrate, tin (II)gluconate, tin (II) tartrate, tin (II) lactate, tin (II) succinate, tin(II) sulfamate, tin (II) formate, and tin (II) silicofluoride.

Non-limiting examples of the second tin salt (tin salt (IV)) includesodium stannate and potassium stannate.

In particular, tin (II) alkanesulfonate, such as tin (II)methanesulfonate, and organic tin (II) sulfonate, such as tin (II)alkanolsulfonate such as tin (II) isethionate are preferably used.

To reduce burning, the concentration of tin salt as Sn²⁺ is preferably 5g/L or higher and more preferably 10 g/L or higher. To improve thestability of plating solution and reduce precipitation, theconcentration of the tin salt is preferably 120 g/L or lower and morepreferably 90 g/L or lower. This concentration also helps reduce cost.

Tin salt having a low lead (Pb) concentration of 1.0 ppm or lower may beused as the tin salt. Using tin salt having a low Pb concentrationallows the plating solution to have a lower Pb concentration.

The electrolytic tin plating solution according to this embodiment maycontain any one of an inorganic acid, an organic acid, and soluble saltsof these acids. Adding the acid or the soluble salt to the platingsolution allows the pH of the surface of a plated object and the pH ofthe surface of Sin forming the Sn deposit to be kept constant, thusproviding a uniform surface potential. This can retard the eutectoidreaction of Pb.

Non-limiting examples of the acid or the soluble salt of the acidinclude a sulfuric acid, a hydrochloric acid, a nitric acid, aphosphoric acid, a sulfamic acid, an organic sulfonic acid (analkanesulfonic acid, such as a methanesulfonic acid, or analkanolsulfonic acid, such as an isethionic acid), and a carboxylic acid(an aromatic carboxylic acid, an aliphatic saturated carboxylic acid, oran amino carboxylic acid). If necessary, a neutralized salt of any oneof these soluble salts may be used. In particular, a methanesulfonicacid is preferably used for ease of handling.

To improve the stability of the plating solution and reduce theprecipitation, the concentration of the acid or the soluble salt of theacid is preferably 50 g/L or higher and more preferably 100 g/L orhigher. This concentration also helps substantially prevent anappropriate Pb deposition potential. To reduce cost, the concentrationof the acid or the soluble salt is preferably 500 g/L or lower, morepreferably 300 g/L or lower, and still more preferably 200 g/L or lower.

The electrolytic tin plating solution according to this embodiment maycontain a surfactant. One or more selected from an anionic surfactant, acationic surfactant, and a nonionic surfactant may be used as thesurfactant. In particular, a nonionic surfactant is preferably used, andan alkylene oxide-based surfactant is more preferably used. Adding thesurfactant to the plating solution allows the surface of a plated objectand the surface of a Sn crystal forming a film to have a uniform currentdensity, thus maintaining a uniform deposition potential at the surface.This can retard the eutectoid reaction of Pb.

Non-limiting examples of the alkylene oxide-based surfactant includepolyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether,polyoxyethylene alkyl amine, polyoxyethylene alkyl amide,polyoxyethylene fatty acid ester, polyoxyethylene polyhydric alcoholether, an ethylene oxide-propylene oxide block copolymer compound, anethylene oxide-propylene oxide random copolymer compound, and apropylene oxide polymer compound. In particular, polyoxyethylene alkylphenyl ether is preferably used.

The concentration of the surfactant is preferably 0.05 g/L or higher,and more preferably 0.5 g/L or higher. Even if plating is performed at ahigh current density to shorten the plating time, the surfactant havingsuch a concentration or higher can reduce burning at an area having ahigh current density. To reduce color irregularities arising fromblackening of the Sn deposit, the concentration of the surfactant ispreferably 100 g/L or lower.

The electrolytic tin plating solution according to this embodimentcontains an acid or a soluble salt thereof and a surfactant. The acid orthe soluble salt thereof is more preferably one or more acids or solublesalts thereof selected from a sulfuric acid, a hydrochloric acid, anitric acid, a phosphoric acid, a sulfamic acid, an organic sulfonicacid, a carboxylic acid, or salts of these acids. The surfactant is morepreferably selected from one or more surfactants selected from ananionic surfactant, a cationic surfactant, and a nonionic surfactant.

The electrolytic tin plating solution according to this embodiment maycontain an organic solvent, an antioxidant, and a chelating agent.Non-limiting examples of the organic solvent include monohydricalcohols, such as methanol and 2-propanol, and dihydric alcohols, suchas ethylene glycol, diethylene glycol, and triethylene glycol.Non-limiting examples of the antioxidant include catechol, hydroquinone,4-methoxyphenol, and ascorbic acid. Non-limiting examples of thechelating agent include oxalic acid, succinic acid, malonic acid,glycolic acid, gluconic acid, gluconolactone, glycine,ethylenediamine-acetic acid, pyrophosphoric acid, and tripolyphosphoricacid.

To form a Sn deposit using the electrolytic tin plating solutionaccording to this embodiment, the pH of the plating solution ispreferably strongly acidic. The temperature at which the Sn deposit isformed should not be specifically limited. However, the temperature ispreferably from 25° C. to 40° C. The current density at which the Sndeposit is formed is preferably from 1 A/dm² to 20 A/dm², and morepreferably from 2 A/dm² to 6 A/dm².

The electrolytic tin plating solution according to the presentdisclosure can be used, for example, to form plated bumps on asemiconductor chip. To form a plated bump, a Sn deposit having apredetermined size is formed at a predetermined position, and then areflow process is performed. The reflow process should not bespecifically limited, but may be performed using a known reflowapparatus.

EXAMPLES

The electrolytic tin plating solution according to the presentdisclosure will now be described in more detail with reference toexamples. The following examples are illustrative, and are not intendedto limit the present disclosure.

Formation of Sn Deposit

A substrate was electrolytically plated with Ni (an electrolytic nickelplating solution: Thru-Nic AMT, manufactured by C. Uyemura & Co., Ltd.,liquid temperature: 50° C., current density: 1 A/dm², plating time: 10min). A Sn deposit was formed on the Ni surface, where an electrolytictin plating solution having a predetermined composition was set to havea liquid temperature of 30° C. and a current density of 4 A/dm².

Estimation of Grain Size

The grain size of Sn grains forming the resultant Sn-plated film wasmeasured by an electron emission scanning electron microscope(JSM-7800F, manufactured by JEOL Ltd.). An IPF mapping image obtainedunder conditions of an accelerating voltage of 20 kV and an illuminationcurrent of 13 nA was analyzed to calculate a distribution range of thegrain sizes of the Sn grains.

Evaluation of Void

The obtained Sn deposit was reflowed at 260° C., and then the presenceor absence of a void was evaluated by an X-ray nondestructive testingsystem (XD7600NT Diamond FP, manufactured by Nordson DAGE). The X-raynondestructive testing system had a tube voltage of 60 kV and an outputof 1.5 W.

First Example

As a heterocycle-containing unsaturated aldehyde compound,3-(2-furyl)acrolein was used. The concentration of theheterocycle-containing unsaturated aldehyde compound was 0.2 mmol/L. Tin(II) alkanesulfonate as a tin salt, methanesulfonic acid as an orgaincacid, and polyoxyethylene bisphenol A ether as a surfactant were added.The Tin (II) alkanesulfonate was added such that the concentrationthereof was 70 g/L as Sn(Sn²⁺), and the methanesulfonic acid, and thepolyoxyethylene bisphenol A ether in the compound were respectively 100g/L, and 50 g/L. The resultant plating solution was set to have a liquidtemperature of 30° C. and a current density of 4 A/dm².

Grain size of Sn deposit obtained had a maximum grain size of 8 μm and aminimum grain size of 1 μm. No voids were observed in the Sn depositafter reflow.

Second Example

A second example was similar to the first example except that(4-pyridyl)acrolein was used as a heterocycle-containing unsaturatedaldehyde compound and that the concentration of the (4-pyridyl)acroleinwas 0.8 mmol/L.

Grain size of Sn deposit obtained had a maximum grain size of 7 μm and aminimum grain size of 1 μm. No voids were observed in the Sn depositafter reflow.

Third Example

A third example was similar to the first example except that1,3-benzodioxole-5-acrolein was used as a heterocycle-containingunsaturated aldehyde compound and that the concentration of the1,3-benzodioxole-5-acrolein was 0.4 mmol/L.

Grain size of Sn deposit obtained had a maximum grain size of 6 μm and aminimum grain size of 1 μm. No voids were observed in the Sn depositafter reflow.

First Comparative Example

A first comparative example was similar to the first example except thatno heterocycle-containing unsaturated aldehyde compound was added.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm anda minimum grain size of 4 μm. Voids were observed in the Sn depositafter reflow.

Second Comparative Example

A second comparative example was similar to the first example exceptthat 1.0 mmol/L of benzaldehyde was added instead of aheterocycle-containing unsaturated aldehyde compound.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm anda minimum grain size of 3 μm. Voids were observed in the Sn depositafter reflow.

Third Comparative Example

A third comparative example was similar to the first example except that1.0 mmol/L of cinnamaldehyde was added instead of aheterocycle-containing unsaturated aldehyde compound.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm anda minimum grain size of 3 μm. Voids were observed in the Sn depositafter reflow.

Fourth Comparative Example

A fourth comparative example was similar to the first example exceptthat 0.4 mmol/L of acrylic acid was added instead of aheterocycle-containing unsaturated aldehyde compound.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm anda minimum grain size of 3 μm. Voids were observed in the Sn depositafter reflow.

Fifth Comparative Example

A fifth comparative example was similar to the first example except that0.8 mmol/L of acrolein was added instead of a heterocycle-containingunsaturated aldehyde compound.

Grain size of Sn deposit obtained had a maximum grain size of 12 μm anda minimum grain size of 3 μm. Voids were observed in the Sn depositafter reflow.

TABLE 1 Examples Comparative Examples 1 2 3 1 2 3 4 5 Heterocycle-3-(2-furyl) (4-pyridyl) 1,3-benzodioxole- None (benz- (cinnam- (acrylic(acrolein) containing acrolein acrolein 5-acrolein aldehyde) aldehyde)acid) Unsaturated Aldehyde Compound (mmol/L) 0.2 0.8 0.4 — 1.0 1.0 0.40.8 Grain Size 1-8 1-7 1-6 4-12 3-12 3-12 3-12 3-12 (μm) Void None NoneNone Formed Formed Formed Formed Formed

Table 1 summarizes the conditions and result of each of the examples andcomparative examples. Using the electrolytic tin plating solutioncontaining the heterocycle-containing unsaturated aldehyde compoundreduced the grain size, and allowed for formation of uniform bumpswithout any void.

The electrolytic tin plating solution according to the presentdisclosure allows for formation of a Sn deposit having a uniform andsmall grain size, and is useful for forming bumps, for example.

What is claimed is:
 1. An electrolytic tin plating solution comprising:a compound serving as a source of supply of tin ions; and an unsaturatedaldehyde compound having a heterocyclic group.
 2. The electrolytic tinplating solution of claim 1, wherein the heterocyclic group is a five-or six-membered heterocyclic group containing at least one of an oxygenatom, a nitrogen atom, or a sulfur atom.
 3. The electrolytic tin platingsolution of claim 1, wherein a content of the unsaturated aldehydecompound having the heterocyclic group in the electrolytic tin platingsolution is from 0.01 mmol/L to 10 mmol/L.
 4. The electrolytic tinplating solution of claim 1, wherein grains to be deposited have amaximum grain size of 10 um or smaller.
 5. The electrolytic tin platingsolution of claim 1, wherein the heterocyclic group is one, two, or moregroups selected from a pyrrolidine group, a pyrrole group, atetrahydrofuran group, an oxolane group, a furan group, atetrahydrothiophene group, a thiolane group, a thiophene group, animidazole group, a pyrazole group, an imidazoline group, an oxazolegroup, a thiazole group, a thiazolidine group, a triazole group, atetrazole group, a dioxolane group, an oxadiazole group, a thiadiazolegroup, a piperidine group, an azinane group, a tetrahydropyran group, anoxane group, a tetrahydrothiopyran group, a pyridine group, a pyranegroup, a thiopyran group, a pyrimidine group, a pyrazine group, apyridazine group, a thiazine group, a morpholine group, a dioxane group,a dithiin group, a thiomorpholine group, a trithiane group, a dithiazinegroup, a thiazepine group, an indole group, an isoindole group, anindolizine group, a benzimidazole group, a benzotriazole group, a purinegroup, a quinoline group, an isoquinoline group, a quinazoline group, aquinoxaline group, a cinnoline group, a phthalazine group, a chromenegroup, an isochromene group, a benzodioxole group, a benzodioxan group,a benzoxazole group, a benzothiazole group, a pteridine group, aphenothiazine group, a phenanthridine group, and a thianthrene group. 6.The electrolytic tin plating solution of claim 1, wherein theunsaturated aldehyde compound having the heterocyclic group is one, two,or more selected from 3 -(2-furyl)acrolein,2-methyl-3-(2-furyl)propenal, 3-(5-nitro-2-furyl)acrolein,(4-pyridyl)acrolein, 1,3-benzodioxole-5-acrolein,3-[3-(4-fluorophenyl)-1-isopropylindol-2-yl]acrolein,5-hydroxytetrahydrofuran-2-acrolein, 5-(2-furyl)-2,4-pentadienal,2-formyl-3-(2-furyl)propenenitrile, 2-cyano-3-(2-furyl)propenal,3-(4-pyridyl)propenal, pyridine-3-propenal, 3-(1H-indole-3-yl)propenal,3-(3,4-diethyl-2-pyrrolyl)propenal, 3-(2-thienyl)acrolein, and3-(isoindoline-2-yl)propenal.